The invention relates to protective lubricant emulsion compositions for web offset lithographic printing.
Web offset lithographic printing is a high speed, high volume process for printing on a continuous paper roll, referred to as a "web". Heat-set web printing is a variation of the offset lithographic printing process, and is employed in commercial printing of medium to high quality work. The inks used in heat-set web printing are specially formulated to provide high gloss and a high quality appearance on the printed sheet. Such inks require that a protective lubricant be applied after the oven curing and chilling stages. This protective lubricant aids in final sheeting and finishing. It prevents marring of the ink surface, and marking or streaking of the partially cured ink.
During heat-set web printing, the desired image is printed on the web with a viscous heat-set ink. The printed web then travels through an oven wherein it is heated to temperatures of 200.degree. F. to 300.degree. F. The oven temperature is typically 300.degree. to 500.degree. F. to achieve this web temperature. Typically, any point on the web traverses the oven in less than a second. The application of heat evaporates ink solvents and is the first step of the multi-step ink drying process. The application of heat to the paper web also has the undesirable effect of removing necessary moisture from the paper.
Upon exiting the oven, the heated web travels over a series of large diameter, water cooled rollers that are incrementally chilled to gradually lower the temperature of the web to about 75.degree. F. This chilling process sets, or dries, the ink surface. At this point the ink is about 80% to 90% set or dried. The web then travels over an applicator device that applies a dilute aqueous lubricant emulsion, commonly a polydimethylsiloxane (silicone) emulsion, over the entire web surface. An example of such an applicator device is the "Automatic Silicone Applicator" made by Ryco Graphic Manufacturing, Inc. This aqueous lubricant emulsion serves to restore moisture to the paper web and to lubricate the ink surface on the printed web. The printed web then travels over surfaces which it directly contacts, such as angle bars, turn bars and the surface of a metal nose cone whereby the printed web is folded. Without the application of a protective aqueous lubricant after the heating and chilling stages, the ink would mar or streak on the printed web as it travels over these surfaces. The aqueous lubricant emulsion also prevents ink build-up on surfaces which could further damage the appearance of the printed web. Moisture is also applied to the web via this aqueous lubricant emulsion, which serves to replace moisture the paper loses during heating in the oven.
Restoration of moisture prior to finishing, bindery and trimming is important because dried paper is dimensionally unstable and brittle. Dried paper is also hygroscopic and will absorb moisture from the surrounding air. Dimensions of paper sheets change as moisture is absorbed. Such changes in paper dimension can ruin a final printed article when they take place after bindery and trimming of printed sheets. The result is a printed article with uneven and/or wavy edges. This is due to differential expansion of paper sheets, commonly referred to as "signatures", after the confinement of being bound together, and subsequent reabsorption of moisture in the form of ambient humidity. Restoration of moisture by the current process and with the material or emulsion described in detail below precludes these problems. Moisture restoration also improves ductility of paper. This helps reduce cracking of paper when it is folded in finishing or bindery.
Silicone lubricant emulsions are commonly used to lubricate and remoisten the paper web. Silicone lubricant emulsions are supplied to the printer with approximately 20 to 60% (wt) polydimethylsiloxane (silicone oil) content. The remaining portion is water, emulsifiers, and optional anti-static agents. The viscosities of silicone oils used in preparation of such emulsions are typically 300 to 500 centistokes. These silicone oil emulsions are diluted with water by the printer prior to use. Typical dilutions range from 1% to 10% of the 60% silicone oil emulsion with 99% to 90% water. The lower wt % silicone emulsions are used in proportionally greater quantity when diluted with water. Dilution is necessary because a minimal amount of emulsion must be used to avoid imparting excess slip properties to the web/ink surface. The amount of slip imparted to the web is difficult to control even with very low dilution ratios of the silicone lubricant emulsion to water.
The silicone emulsions commonly used as protective lubricants have many advantages and disadvantages. They are less prone to re-solvate the heat-set ink. Further, they impart a very low coefficient of friction (slip) to the printed web. However, such silicone emulsions are disproportionately costly, and frequently impart too much slip (low coefficient of friction) to the printed web. Silicone emulsions are often incompatible with recycled papers and can aggravate paper cracking problems which occur during the binding process. Dilute silicone emulsions do not apply well to the web with currently used application equipment. Silicone oils also inhibit the adhesion of adhesive to the paper, which is commonly applied in bindery and finishing processes. The foregoing disadvantages encountered using known lubricants have driven a search for alternatives.
Alternative formulations have included ingredients like waxes, mixed aromatic and aliphatic hydrocarbon oils, vegetable oils, and cationic surfactants. Such wax, oil, and cationic surfactant-based alternatives may also contain volatile organic compounds. The use of wax, oil and cationic surfactant emulsions alone or as diluents for silicone lubricant emulsions has been largely unsatisfactory. Such unsatisfactory performance may be due to any of the following: build-up of the emulsion on equipment surfaces; insufficient slip; poor application characteristics; ink streaking and marring; loss of gloss; incompatibilities with silicone emulsions; and instability upon dilution. This latter instability is manifest in the rapid physical separation upon dilution of such alternative lubricant emulsions with water. Acceptable processing requires that the lubricant emulsion remain uniformly dispersed and stable in this very dilute solution, as uniform dispersion is essential to uniform application. Even blends of silicone emulsion with wax, oil and/or cationic surfactant emulsions have proven to be unstable on dilution, resulting in marring and streaking problems during the printing process.
One attempt to forego some of these problems involves the use of mixed emulsions, containing one or more polydimethylsiloxanes, mineral oil, a nonionic emulsification agent and water. The mixed emulsions are applied to wet ink just after printing and prior to cure or heat treatment of the printed web. U.S. Pat. No. 5,460,856 discloses this type of mixed silicone oil emulsion. The compositions set forth in this patent, however, require unusual application equipment, as the lubricant emulsion is required to be applied to wet ink prior to curing or heat treatment. Due to the application of the emulsion so early during the process, the viscosity and dilution factors of the emulsion are suited to the processing yet to be undergone, as opposed to that of emulsions intended for application further on in the processing. Therefore, the viscosity and dilution factors of this composition make it unsuitable for application from standard machinery. This method is a significant departure from standard processing and the current state of the art. Further, compositions such as the compositions set forth in the 856 patent require that silicone oil and mineral oil be added slowly to a mixture of water and emulsifier to form the resulting mixed emulsion compositions. Such compositions are less stable upon dilution with water to concentrations used in standard processing. This instability is a result of the method used in the 856 patent to make the mixed silicone oil/mineral oil emulsion.
It is, therefore, one object of the invention to provide a lubricant, applicable to the cured web with standard equipment, which provides enhanced dilute stability, lubrication, anti-static and anti-smudge properties to the cured web.
It is another object of the invention to provide a novel and improved non-silicone oil aqueous emulsion for use as a protective paper lubricant in the applicator equipment of offset lithographic heat-set web presses.
A further object of the invention is to provide a lubricant emulsion that is non-hazardous and contains no volatile organic compounds.
Yet another object of the invention is to provide an economical and universally compatible lubricant emulsion that may be used as a diluent, extender or total replacement for presently used silicone oil emulsions, thereby providing control of the slip or coefficient of friction of the web surface. The disclosed invention is universally compatible with silicone oil emulsions, and therefore can be blended with the same to tailor performance properties, for example, coefficient of friction and economics.
Another object of the invention is to provide a non-silicone lubricant emulsion that is universally compatible with all paper stocks including papers of high recycled content.
Another, and most important, object of the invention is to provide improved application characteristics of aqueous lubricant emulsions, and silicone emulsions modified therewith. The present invention provides a more uniform wet coating across the applicator roller during application of the lubricant emulsion to the paper web.
These and other objects of the invention will become clear to the skilled artisan upon reading and understanding the following disclosure.